1. Field of the Invention
The present invention relates to an optical pickup device employing hologram which is used in an optical disk device and the like.
2. Description of the Prior Art
An optical pickup device uses laser beam to record information in an optical recording disk such as an optical disk or read the information therefrom or detect servo signals. A prior art pickup device for a compact disk, for example, includes a semiconductor laser as a light source, a collimator lens for converting light emitted from the semiconductor laser into parallel beams, an objective lens for converging the parallel beams on the optical disk, a beam splitter for partially separating light reflected from the optical disk, a cylindrical lens for astigmatically converging light separated by the beam splitter, a detector receiving converged astigmatical light for detecting a recorded signal and a servo signal, and a photodetector for monitoring the power of laser beam which is partially reflected by the beam splitter on forward path from the semiconductor laser to the optical disk.
In this optical pickup device, however, the increasing number of components including the beam splitter leads to complicated device structure, and therefore, in recent years various optical pickup devices in which the beam splitter and the like are replaced with a hologram have been developed (see, for example, Japanese Unexamined Patent Publication No. 76035/1991 (G11B 7/135)).
FIG. 28 is a schematic diagram showing an optical pickup which employs the prior art hologram. The optical pickup has a first hologram 103 as a diffraction grating for splitting a laser beam on an optical path into three beams and a second hologram 104 for converting light reflected from an optical disk 101 so that the resultant wave front is astigmatical and then converting the light. Laser light from a semiconductor laser 102 emitting a laser beam upward is split into three beams by a first hologram 103, and the beams passing through a second hologram 104 is focused on the optical disk 101 by a focusing lens 105. A feedback beam obtained by reflecting the light from the optical disk 101 and converging the light with the wave front being astigmatical is detected by a photodetector 106 as a signal sensor.
In this optical pickup device, there is no loss of light since the feedback beam transmitted from the optical disk 101 and received by the light emitting element 106 is not diffracted by the first hologram 103, and the device can be miniaturized as compared with a device employing other elements like a prism since the second hologram 104 makes diffraction of the light leading to the photodetector 106 used as a signal sensor.
In the above-mentioned device, however, although it is necessary that the first hologram 103, the second hologram 104 and the photodetector 106 must be arranged in position and assembled cooperatively. Therefore, construction of these components is somewhat laborious, and adjustment in position among these components is also tedious work.
U.S. Pat. No. 5,018,804 discloses an optical pickup device which includes a diffraction grating for changing a direction of a laser beam emitted by a semiconductor laser at a right angle and a six-divided light detecting element for signal detection provided in a lower side position of the diffraction grating, so that the device can become thinner while grating intervals among hologram elements can be broadened.
However, since the components in this device are also independent of one another, there still remain the problems that construction of the components is laborious and that adjustment in correlated positions of those components is tedious work.
In addition to that, the prior art device has a disadvantage that since wire bonding pads of the semiconductor laser and the photodetector for signal detection are orthogonal in position or have a large difference in height to each other, wire bonding cannot be performed simultaneously to both of those elements.